In recent years, in order to remove harmful substances such as a hydrocarbon (HC) compound, carbon monoxide (CO) and carbon nitride (Nox), which are contained in exhaust gas exhausting from an internal combustion engine, an exhaust gas purifying catalyst in which particles of noble metal such as platinum (Pt) are supported on a support of a metal oxide such as alumina (Al2O3) has been being widely used. In the conventional exhaust gas purifying catalyst, a large amount of such noble metal particles are used in order to enhance durability of the noble metal particles against variations of an ambient atmosphere. However, it is not desirable to use the large amount of noble metal particles from a viewpoint of protecting the earth's resources.
From such a background, an attempt has been recently made to enhance the durability of the noble metal particles in such a manner that transition metal such as cerium (Ce) and manganese (Mn), or a transition metal compound, which functions as an oxygen storage component (OSC) material, is disposed in the vicinity of the noble metal particles by an impregnation method, whereby the variations of such an atmosphere around the noble metal particles is suppressed by the transition metal or the transition metal compound (refer to Patent Documents 1 to 4). Note that, in accordance with the method as described above, enhancement of activity of the noble metal particles can also be expected in addition to the enhancement of the durability of the noble metal particles.
Patent Document 1: Japanese Patent Laid-Open Publication No. H8-131830
Patent Document 2: Japanese Patent Laid-Open Publication No. 2005-000829
Patent Document 3: Japanese Patent Laid-Open Publication No. 2005-000830
Patent Document 4: Japanese Patent Laid-Open Publication No. 2003-117393